1. Field of the Invention
The present invention relates to a carrier for two-component electrophotographic developer used in copying machines and printers and to an electrophotographic developer containing the carrier.
2. Description of Related Art
A two-component developer used for developing an electrostatic latent image in electrophotography comprises a toner and a carrier. The carrier is mixed and agitated with the toner in a development box to give a desired charge to the toner and carries the charged toner onto an electrostatic latent image formed on a photosensitive material (photoreceptor) to form a toner image.
The carrier remains on the magnet and is returned to the development box where it is again mixed and agitated with fresh toner particles for repeated use.
In order to maintain high image quality over a service life of a developer in a stable manner, the carrier is required to have stable characteristics over the life.
Many of the state-of-the-art carriers for two-component dry developers for electrostatic latent image development have a resin coat for obtaining high image quality.
Because a developer is always under the stress of collisions among the particles or with the wall of a development box or a photoreceptor, etc. during the service life, the resin coat comes off the carrier core by the collisions. Further, the toner adheres to the surface of the carrier particles, that is, so-called spent phenomenon due to the heat generated by the collisions. It follows that the carrier characteristics are deteriorated with time, and it eventually comes necessary to exchange the developer for a new one.
In order to prevent deterioration of carrier characteristics, studies have been made on the resin to be used to coat the surface of a carrier. Of various resins proposed to date a silicone resin having a low surface tension has now been prevailing.
Carriers comprising a carrier core such as ferrite powder coated with a silicone resin and an electrophotographic developer containing such a silicone-coated carrier undergo great changes in chargeability and fluidity with environmental changes. In particular, the charge quantity decreases under a high temperature and high humidity condition to cause fog and increase of toner scattering, and the charge quantity increases in a low temperature and low humidity condition to cause a reduction in image density. Further, the fluidity varies with the environmental changes to impair conformity to a toner concentration sensor, which may lead to fatal image defects.
A phenylsilicone resin and a methylphenylsilicone resin have been proposed as a coating resin for stabilizing the chargeability of a carrier as disclosed in Japanese Patent Laid-Open Nos. 59238/89 and 264563/92, but they fail to bring about a solution to the above problem and have an additional disadvantage that the image characteristics largely vary during the service life.